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This is interesting. Can it work with any website, even say document repositories hosted on standard servers like gitbook?

It works with pretty much any website, and works well with docs hosted on GitBook yes, I have embedded a website that's hosted there.

Confirmation email doesn't work, so cannot try it. I attemtped twice and checked spam

Apologies, please email me at support at embedding.io. If you have something you'd like embedded, please also mention it so I can set it up for you.